1. Technical Field
The embodiments herein generally relate to CFO estimation in receivers, and, more particularly, to scheme for generating hypothesis and estimating a carrier frequency in a receiver based on the hypothesis.
2. Description of the Related Art
In a wireless receiver, such as a WCDMA receiver, the inaccuracies of a local oscillator results in a frequency mismatch with that of carrier frequency of a received signal. This frequency mismatch and carrier frequency offset (CFO) degrades the performance of a receiver. This is due to usage of less expensive local oscillator in the receiver. To estimate the CFO, a radio frame from a transmission signal in a time domain is divided into equal-duration time slots. In each of the time-slot, data and/or control messages are transmitted as a sequence of symbols having a fixed length. Each time slot may be further sub-divided into two or more sub-slots.
FIG. 1 illustrates a conventional method of receiving an nth radio frame from a transmission signal and dividing the radio frame into ‘m’ equal-duration time slots. A sequence of length ‘N’ of a radio-frame (e.g., a pilot sequence) stored in a memory at the receiver end is also divided into ‘m’ sub-sequences of equal length and correlated with corresponding reference symbols (or two or more sub-slots) to obtain ‘m’ correlation values (e.g., correlation value 1, correlation value 2, upto correlation value m, etc.).
FIG. 2 illustrates a conventional method of estimating a carrier frequency offset using a Fast Fourier Transformation (FFT) technique. In step 202, a sequence that includes an nth radio frame is received as depicted in FIG. 1. The nth radio frame is illustrated in accordance with an equation:{rn(j): j=0 . . . N−1}  (i)In step 204, each (N/m) symbol is correlated with a corresponding reference symbol to obtain ‘m’ correlation values which is illustrated in accordance with an equation:{an,i, i=0 . . . m−1}  (ii)In step 206, an FFT of length K is computed on {an,i} (e.g., a first sequence) in accordance with an equation:
                                          A            ⁡                          (              k              )                                =                      FFT            ⁡                          (                              {                                  a                                      n                    ,                    i                                                  }                            )                                      ,                  k          =                                    0              ⁢                                                          ⁢              …              ⁢                                                          ⁢              K                        -            1                                              (        iii        )            
Similarly, an FFT is computed on every sequence that is received at the receiver. In step 208, A(k) is added to hypothesis H(k), and is illustrated in accordance with an equation:H(k)=H(k)+A{k}  (iv)In step 210, a peak position is determined in the hypothesis and illustrated in accordance with an equation:ρ=argmax H(k), kε(0, 1 . . . K−1)  (v)
In step 212, a carrier frequency is estimated from the peak position ‘ρ’. As depicted in this method, the received sequence is split into multiple sub-sequences of equal, shorter lengths and correlated with the corresponding reference symbols. Hypothesis is constructed by computing DFT over this sequence of correlation values. Hypothesis averaging is done in a frequency domain by adding hypothesis of multiple radio frames (e.g., for multiple sequences) to obtain averaged hypothesis. The frequency corresponding to the peak position in the averaged hypothesis gives a CFO estimate. Since, the FFT is performed for each received sequence (or radio frame), the method results in high computational complexity.
FIG. 3 is a conventional method of estimating a carrier frequency offset using an inner slot differential combining (ISDC) technique. In step 302, a sequence that includes an nth radio frame is received as depicted in FIG. 1. The nth radio frame is depicted as the same nth radio frame in the conventional method of FIG. 2, and is illustrated in accordance with an equation:{rn(j): j=0 . . . N−1}  (1)In step 304, each (N/m) symbol is correlated with a corresponding reference symbol to obtain ‘m’ correlation values which is illustrated in accordance with an equation:{an,i, i=0 . . . m−1}  (2)
In step 306, a correlation values are multiplied with a complex conjugate of a previous correlation value as shown in FIG. 3. For example, if a1, a2, a3 and a4 are the correlation values, the multiplication operation is performed as: a=a2*a1, a3*a2, a4*a3. In step 308, an,i a*n,i-1 are added to hypothesis h in accordance with an equation:
                    h        =                  h          +                      (                                          a                                  n                  ,                  i                                            ⁢                              a                                  n                  ,                                      i                    -                    1                                                  *                                      )                                              (        1        )            In other words, the hypothesis is generated by multiplying (i) a2 with a complex conjugate of a1, (ii) a3 with a complex conjugate of a2, and (iii) a4 with a complex conjugate of a3. In step 308, a carrier frequency offset is estimated based on the argument of h. As depicted from the FIG. 3, ISDC technique is a low complexity CFO estimation technique. It is also depicted that the CFO estimation is performed in time domain, and hence the CFO estimated leads to a noise enhancement in the WCDMA receiver. To counter the effects of noise-enhancement, ISDC technique needs relatively a large number of averages which leads to a larger number of computations.
Additional, using an expensive local oscillator to estimate the CFO will lead to a trade-off in cost, number of components in the receiver. Thus, there is key challenge to balance the strike between quality, performance, computations, components costs, and functionality of the receiver. Accordingly there remains a need to accurately estimate a carrier frequency offset and correct it in the receiver without having to increase the computations, the cost, and at the same time balance the performance of the receiver.